This present invention relates to high-speed, high precision measurement of the distance between two near contact surfaces one of which is an optical transparent element. In particular the invention relates to an apparatus and method for performing this measurement using a heterodyne interferometer. Moreover, this invention relates to an optical interferometer for measuring the flying height of a magnetic head.
In magnetic data storage, there is a requirement for measuring the flying height of the slider assembly which is in near contact with a high-speed rotating disk in order to optimize the performance of the slider assembly. The flying height is the distance between the magnetic head pole and the surface of the rotating magnetic disk. Recently, the rapid increase in recording density has caused a continuous reduction in the flying height. The flying height is generally less than 250 nm depending on the design of the slider. The trend is toward an ultra-low flying height which is less than 25 nm.
Optical Flying Height Testing (OFHT) is the most popular testing technique in the field. OFHT are almost invariably based on interferometry. There are two known methods. The first method is measuring the flying height from a real magnetic disk through the backside of the slider. The second method is measuring the disk-slider spacing directly, using a transparent disk to replace the magnetic disk. The first method is not popular due to the fact that the accuracy of the measurement depends on the uniformity of the thickness of the slider. Moreover, the backside of the slider is not accessible on most production slider assemblies. Currently, the second method is more widely used in commercial applications.
Until now, there were three types of interferometry developed for flying height testing. The three types include white light interferometry, three-wavelength interferometry, and monochromatic interferometry. With a white light interferometer, a flying height of 700 to 300 nm can be detected accurately. Three-wavelength interferometer is an extension of white light interferometer. A flying height of less than 250 mm can be measured with a three-wavelength interferometer with acceptable errors. Both a white light interferometer and a three-wavelength interferometer are subject to multi-reflections in the disk-slider air bearing, which produces errors. When the flying height is reduced to 50 nm, the error caused by multi-reflections cannot be neglected. Therefore, the polarization interferometer was developed to measure ultra-low flying height.
In U.S. Pat. No. 4,606,638, Sommargren proposed a polarization phase modulated Fizeau interferometer in which the reference surface is the front surface of a plate polarizer. The modulated interference pattern is detected by a CCD array. The signal provides the absolute distance between the reference surface and the flying slider. Although this technique was an advance in providing the flying height, pitch angle and roll angle in a single measurement, it is not practicable because the measurement accuracy is dependent on the flatness of the glass disk surface which acts as a plate polarizer.
In U.S. Pat. No. 5,218,424, Sommargren proposed a polarization interferometer. In a polarization interferometer, a coherent, single wavelength, linearly polarized beam passes through a phase shifter which varies the relative phase of the orthogonal polarization components of the beam by 0, xcfx80/2, xcfx80, 3xcfx80/2. Then the beam is directed to the glass disk at a Brewster""s angle. The polarized beam P will pass through the glass disk and strike the slider without being reflected from glass disk surface. While the S polarized beam will be reflected by each glass-air interface. Therefore, the S polarization beam can be used as the reference beam and the phase difference between S and P polarization beams carries the flying height information. The phase change which is caused by the optical path difference between S and P polarization beams, therefore, can be determined by a four-step algorithm. By using the polarized laser beam, the error caused by multi-reflection is completely eliminated. Thus, the apparatus is capable of measuring extremely small gaps. Despite the advantage, this technique has some significant limitations, including the use of a very expensive complicated, high speed phase modulator. Moreover, the data processing rate is quite slow, around 15 HZ, which is not suitable for commercial applications.
In U.S. Pat. No. 5,660,441, Groot proposed a quad-beam polarization interferometer. Three of the four beams are S polarization, the other one is a P polarization. These four beams are parallel to each other and are directed to the glass disk at Brewster""s angle. Except for the P polarized beam passing through glass disk without reflection, all the beams are reflected back from the surface of the glass disk. After being reflected from the slider ABS, the P polarized beam again passes through the glass disk. The two reflected S polarized beams are made to interfere thereby providing a reference. The P polarized beam is made to interfere with the remaining S polarized beam. Thus, the interfered beam contains the flying height information. The spacing between disk surface and slider ABS can be detected by comparing the two interfered beams. The main advantage of this technique is that it eliminates the phase modulator which was used by all the prior-art techniques. Therefore, the signal-processing rate can be very fast, and the error introduced by the phase shifting can be eliminated. This apparatus is capable of measuring ultra-low flying height with the required accuracy. In U.S. Pat. No. 5,751,427, Groot further improved his polarization interferometer by using a single linearly polarized beam.
All these prior art techniques are based on homodyne interferometry, using phase measurement to find out the flying height by detecting the optical path difference between reference beam and measurement beam. One phase measurement method, which is generally employed in all these prior art techniques, is the four-step algorithm. This method is subject to the intensity variation of the laser beam, which may be caused by an unstable laser source, birefringence of the high-speed rotating glass disk, disturbances from the surroundings, etc. Moreover, the reflection of the slider ABS will introduce a phase change in the measurement beam which strikes it. In the prior art technique, this phase change was assumed to be xcfx80. These will cause flying height measurement errors as large as 10 nm. It is acceptable when the flying height is above 250 nm. When flying height is reduced to less than 50 nm, it is necessary to precisely measure the phase change introduced by the ABS reflection. Usually, the correction of the phase change occurring at the ABS will be done by a measurement instrument independent of the flying height tester which is called an ellipsometer. The correction of the ABS reflection phase change is substantial disadvantage of all the prior art techniques. Although Groot integrated the ABS reflection phase change correction with the flying height testing into one instrument in his U.S. Pat. No. 5,757,427, the correction is still a separate measurement step that has to be done before the gap measurement.
Thus, the objective of the present invention is to provide an apparatus and method capable of measuring the flying height by removing various uncertainties in an OFHT using heterodyne interferometry without requiring any extra measurement other than flying height testing. The embodiment of the present invention has a laser source emitting two orthogonally polarized beams of different frequency by using a Zeeman laser source or by using an acousto optic modulator and recombining the beam by using optical components. The beam is sally filtered and expanded in diameter by using a spatial filter, by using a pinhole, or with optical fibers. The beam is made to strike the object at a single point or to scan on the object by using acousto optic deflector. The embodiment of the present invention has means to obtain parallel scanning beam and to focus the scanning beam on a single plane. The beam is divided in to two, one for the reference path and the other for the measurement path. The measurement beam strikes the glass disk at an oblique angle such that the P polarized beam refracts through the glass disk and is reflected from the surface of the object. The S polarization beam is reflected from the bottom surface of the glass disk. The reflected S and P polarized beams from the bottom surface of the disk and the object surface are made to interfere on the measurement photo detector. The reference beams are made to interfere at the reference photo detector. Upon comparison of the phase change between the reference and the measurement signal, the displacement and the distance between the disk surface and the object can be obtained using a phase meter and adopting heterodyne interfrometric principles. The reference signal can also be obtained by reflection from the surface of the object or the bottom surface of the glass disk by directing the beam at a particular oblique angle.
The second embodiment of the present invention is a means to measure an object surface smaller than the focused spot size of the beam by using a diaphragm before the measurement photo detector to allow a smaller portion of the interfered beam to pass through it. The smaller the diameters of the diaphragm or slot results in a smaller object being measured in the focused spot size.